WASHINGTON — The U.S. Postal Service will stop delivering mail on Saturdays but continue to deliver packages six days a week under a plan aimed at saving about $2 billion annually, the financially struggling agency says. In an announcement set for later today, the service is expected to say the Saturday mail cutback would begin in August. Under the new plan, mail would be delivered to homes and businesses from Monday through Friday, and would still be delivered to post office boxes on Saturdays. Post offices now open on Saturdays would remain open on Saturdays. The move accentuates one of the agency’s strong points — package delivery has increased by 14 percent since 2010, officials say, while the delivery of letters and other mail has declined with the increasing use of email and other Internet services. Over the past several years, the Postal Service has advocated shifting to a five-day delivery schedule for mail and packages, and it repeatedly but unsuccessfully appealed to Congress to approve the move. Though an independent agency, the service gets no tax dollars for its day-to-day operations but is subject to congressional control. It was not immediately clear how the service could eliminate Saturday mail without congressional approval.